Edema, the abnormal or excessive retention of fluid at a site in the body, can produce damaging stress on the body and inhibit proper functioning of organs. Edema inhibits blood flow in tissue, raises systemic blood pressure and otherwise impairs healthy body function.
Edema produces swelling which often results in a constriction of blood flow to the affected area. This can place stress on the heart, kidneys, brain, muscle tissue and other organs. Causes of edema include trauma, burns, hypersensitive reactions, thrombophlebitis and disease. Edema can even result from malnutrition, obesity and lack of exercise. In the heart, edema can produce heart failure. Cardiac edema increases the volume of the heart wall; the wall thickens and reduces the volume of the chambers of the heart. Cardiac output is reduced and the workload of the heart is increased. Head trauma often results in edema. Serious head injury is almost always associated with excessive fluid retention in brain tissue and brain swelling. As the brain swells the increase in tissue volume is confined by the rigid cranial cavity. The resulting pressure increase restricts blood supply and, if not relieved, produces brain damage. In muscle tissue edema can produce compartment syndrome. Injury can cause a volume of tissue to retain excess fluid and swell. The volume of the swelling tissue is constrained by surrounding tissue so that blood supply to the tissue is restricted.
When excessive fluid collects in tissue there is a need to mitigate the condition to avoid related adverse physiological effects and as an aid to treatment. Edema is often associated with low blood flow and its attendant problems and can affect any location or organ of the body. As water content of tissue can change from time-to-time, and sometimes rapidly, there is a need for an edema monitor which can detect tissue water content in any tissue or organ of the body and monitor it continuously.
Prior devices for measuring edema, such as those for measuring pulmonary edema, involve injection of indicator directly into the bloodstream. Catheters introduced into the circulatory system deliver the indicator and detect the response. One such device is described in U.S. Pat. No. 4,676,252 issued to Trautman et al. U.S. Pat. No. 4,819,648 to Ko discloses a device for electromagnetically sensing impedance changes in the brain to monitor brain fluid levels.
It is a purpose of this invention to provide a system to quantify the water content or condition of edema in any selected tissue or organ of the body and continuously monitor changes therein.
It is a further purpose of this invention to quantify tissue water content by modeling the relationship between tissue water content and a thermal property of tissue that varies as a function of tissue water content.
It is a purpose of this invention to provide an edema monitor in which a thermal probe is introduced into thermal communication with live tissue at a selected site and energized to transfer thermal energy to the tissue and tissue water content is determined from a thermal property of tissue which varies as a function of tissue water content.
It is an objective of this invention to assess tissue water content as a function of the power used to heat tissue at a selected site and a thermal property of tissue which varies as a function of tissue water content.
It is also an object of this invention to quantify edema in tissue as a function of the power used to heat tissue at a selected site and the thermal conductivity of tissue.
In the present invention, the monitoring of water content (edema) in live tissue is provided by detecting the thermal response of the subject tissue to the application of thermal energy and computing water content as a function of the thermal response and thermal energy or power used. Certain thermal properties of tissue vary as a function of tissue water content. For example, thermal diffusivity and thermal conductivity of the tissue increase as the water content of the tissue increases. Accordingly, the thermal response to the introduction of heat in a selected tissue sample or organ is a function of these properties. Herein, the terms tissue water content and edema are essentially synonymous, edema being a condition of abnormally high tissue water content.
A preferred embodiment, the invention includes a thermal probe which thermally communicates with tissue in contact with it and which electrically communicates with a monitor. The probe incorporates an embedded thermistor. In a minimally invasive probe, a distal thermistor is embedded in the tip of a narrow gage catheter (1-mm diameter). The catheter is inserted into tissue at a site to be examined and effects thermal contact with surrounding tissue. The thermistor, adapted for thermal contact with the tissue, is heated to a small increment above the tissue temperature baseline. (For example the temperature of the thermistor surface may be elevated to a predetermined temperature approximately 2xc2x0 C. above the tissue temperature baseline.) A second or proximal thermistor may be embedded in the probe for monitoring tissue baseline temperature and compensating for baseline temperature fluctuations. The distal thermistor is heated at intervals by power source within a control circuit. The power used to elevate the temperature in an interval is indicative of a value of the selected thermal characteristic, for example, thermal conductivity and/or thermal diffusivity, in tissue at the location of the thermistor. The sensed temperature results in a signal from the power source functionally related to the thermal response in the tissue to the application of heat, which signal is used to calculate a value indicative of tissue water content. The following example is based on thermal conductivity.
When the thermistor is in thermal contact with live tissue at a site where water content is to be assessed, the power dissipated by the heated thermistor (typically within the range of 0.005-0.01 W) provides a measure of the ability of the tissue to carry heat by both conduction in the tissue and convection due to tissue blood flow. In operation, the thermistor is energized and a thermal field propagates into tissue contacting and surrounding the thermistor. The initial propagation of the field is due substantially to inherent tissue conductivity (thermal conductance). Subsequent propagation of the field is affected more by tissue convection (blood flow or perfusion). A monitor or data processor controls the probe, records the data and distinguishes between the effect of the inherent thermal conductivity characteristic of the tissue and convective heat transfer due to tissue blood flow. The inherent or intrinsic thermal conductivity of the tissue at the site of the thermistor is determined from the initial rate of propagation of the thermal field in the tissue, separated from the effects of convective heat transfer.
A data processing technique by which the thermal conductive and convective effects of the heated thermistor are distinguished and separated will now be discussed. Measurements are made under effectively transient conditions, i.e., at times which are short relative to the time required for the system to reach steady state. Accordingly, the temperature change produced in the tissue is permitted to vary in any arbitrarily selected manner with time. The power required to heat the tissue and the resulting temperature change are recorded. An intrinsic thermal conductivity value is calculated using data obtained at an initial time period. The conductivity value is used to assess the fluid content (or edema) of tissue at the site of the probe. The water content of the tissue is computed as a function of the intrinsic thermal conductivity of the tissue and data derived by using a model of the relationship of intrinsic tissue conductivity values to tissue water content values.
As is often the case in monitoring procedures, there is some margin of error that must be held within a range deemed appropriate for acceptable or optimum operation. When direct computation of conductivity (or other thermal property) does not lead to an acceptably accurate calculation of water content, an iterative process may be used to optimize the accuracy of the water content calculation. For example, computation can be based on a thermal model requiring a series of heating cycles with measurements at two or more selected times within each cycle. These measurements occur during a temperature change cycle in which the temperature of tissue at the selected site is raised from a first unperturbed value to a second value and relaxed back to an unperturbed value. A thermal model and related mathematical equations are described in U.S. Pat. No. 4,852,027 to Bowman et al. When data used to assess the thermal conductivity of tissue includes measurements made at least two selected time periods in an overall temperature changing cycle, data processing occurs in an interactive or iterative operation so as to converge relatively rapidly to a final solution for conductivity of tissue at the site of the probe. In one embodiment, the thermistor is energized to heat the tissue at the selected site from an unperturbed temperature value to a second higher temperature value and then permitted to decay, i.e. to cool, to an unperturbed value. Power is applied to energize the thermistor in any appropriate manner that produces an arbitrarily selected change as a function of time in the volume mean temperature of the tissue surrounding the thermistor. Measurements are made in at least two selected time periods during the heating and cooling cycle. The effects of the flow in the tissue (perfusion) on the measurements involved are least (substantially negligible) during the initial stage of the heating cycle and greater during the later portion of the cycle. Particularly, the effects of flow are greater during the cooling portion of the cycle than during the heating portion.
In the iterative computation, the temperature of the thermistor is caused to rise to initiate each heating cycle and relax at the end of each cycle. An initial determination of a value for intrinsic thermal conductivity (or thermal diffusivity), is calculated during a first time period in the initial and each subsequent heating cycle. This first time period calculation is made at the initial stage of each heating cycle. A calculation of the convective heat transfer effect in the tissue due to perfusion of the tissue is separately calculated at a second time period, later in the heating cycle, using the conductivity value obtained in the initial time period and perfusion data obtained at the second time period, the effects of convective heat transfer during the second time period being greater than the convective heat transfer effects during the first time period. The perfusion value obtained at the second time period is used to recompute a second, more accurate value of thermal conductivity in the first time period. The process can be repeated as many times as necessary. In each calculation of perfusion the value of conductivity obtained in the prior calculation is used. Similarly, in each successive computation of thermal conductivity the prior value of perfusion is used. The iterative process will lead to convergence wherein the same value of thermal conductivity is obtained in successive calculations. This value of conductivity can be used to compute the fluid content of tissue at the location of the probe.
The calculation of edema in the above described embodiment thus takes into account the effective thermal conductivity of the subject tissue, that being the convective heat transfer effect produced by tissue perfusion plus the intrinsic thermal conduction of the tissue, and separates the convective heat transfer effect (perfusion) from the intrinsic thermal conductivity. (Similarly, effective thermal diffusivity is the intrinsic thermal diffusivity of the tissue plus the perfusion related diffusivity effects.)